The industrial pollutants known as PCB's were first manufactured in

Dear karant,

I'm happy to respond. The first thing I will say is that student mistakenly believe that the SC GMAT is primarily a test of grammar. In fact, grammar is only one of three equally important strands---the other two are [urlhttps://magoosh.com/gmat/2013/logical-predication-on-the-gmat-sentence-correction/]logic[/url] and rhetoric. What you are asking here is not a matter of grammar: it's a matter of logic.

The simple rule is that GMAT considers the past perfect either questionable or outright wrong when its use is entirely redundant. The GMAT is 100% against redundancy. Therefore, we have to use logic to figure out what's redundant and what's not.

Look at this sentence, the SC question in this thread. This is (A), the OA:
The industrial pollutants known as PCB's were first manufactured in 1929 and were used as coolants for electrical equipment in Europe and North America until the 1970's, when studies showed that the compounds had entered the food chain and were harmful to some animals.
This version has the past perfect, and the use of the past perfect makes very clear that in the 1970s, when the studied were done, the pollution problem had taken place already. The environment was already contaminated by the time when scientists conducted these studies.

Now, change that to the simple past:
The industrial pollutants known as PCB's were first manufactured in 1929 and were used as coolants for electrical equipment in Europe and North America until the 1970's, when studies showed that the compounds entered the food chain and were harmful to some animals.
This changes the meaning. This tells us that no PCB's entered the food chain before the 1970's, but somehow, right when the studied were done, that's when the pollutants suddenly permeated the entire food chain. First of all, that's completely nonsense, but also that's a completely different meaning.

Therefore, the past perfect is required in that question, because removing the past perfect changes the meaning. If you can take something out and the meaning changes, then what you have taken out is not redundant. By definition, something is redundant only when it can be removed without altering the meaning in the least. Once again, the GMAT is 100% against redundancy.

Now, look at that OG SC problem, #89 in the OG 2017. Incidentally, my friend, it is common courtesy to cite the exact number of any question you quote from the OG.

Here's (A), with the past perfect.
His studies of ice-polished rocks in his Alpine home land, far outside the range of present-day glaciers, led Louis Agassiz in 1837 to propose the concept of an age in which great ice sheets had existed in now currently temperate areas.
Let's just keep this same version, but change the past perfect to the simple past:
His studies of ice-polished rocks in his Alpine home land, far outside the range of present-day glaciers, led Louis Agassiz in 1837 to propose the concept of an age in which great ice sheets existed in now currently temperate areas.
Absolutely no change in meaning. Louis Agassiz was doing this work in 1837, and both versions make very clear that the "age" of the "great ice sheets" was well before 1837--in fact, it was back in the most recent Ice Age. Whether we have the past perfect or the simple past, the sentence has exactly the same meaning: by definition, that makes the use of the past perfect tense redundant in this context.

You see, my friend, there is no simple rule about when the past perfect is right or wrong. You have to think about the logic of the situation and ask yourself whether the meaning changes if you change the past perfect to the simple past. Any feature of a sentence that can be removed without changing the meaning is, by definition, redundant.

Does all this make sense?
Mike
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